Electric field-driven interfacial reduction of metal ions in microdroplets: gold, silver, and nickel

A bulk aqueous solution containing 100 μM HAuCl₄ has been shown to spontaneously form gold nanoparticles (Au NPs) in 2-3 days when stored at room temperature. We demonstrate that Au NPs can be spontaneously formed within a few microseconds to milliseconds when the same solution is sprayed in the form of microdroplets (10-30 μm in diameter) using N₂ as the nebulizing gas under ambient conditions. The rapid formation of Au NPs establishes that the air-water interface of microdroplets plays a dominant role. The reduction of metal ions in water microdroplets is driven by electron transfer at the air-water interface of water microdroplets aided by the strong electric field and the lack of three-dimensional solvation at the surface. The reduction of metal is accompanied by the formation of H₂O₂ resulting in part from the recombination of OH˙ produced at the interface. We observed that the size of the Au NPs increases when the distance between the tip and collector increases suggesting the rapid nucleation and growth of Au NPs within the microdroplets. The nanoparticle generation in microdroplets is not limited to Au, and we extend the scope of this method to other metals such as silver (Ag) and nickel (Ni) indicating a minimal role of the metal's position in the electrochemical series. When polar protic solvents such as CH₃OH, and C₂H₅OH replace water as a solvent, Au NPs are seen to be formed but at a much slower rate whereas in acetonitrile (ACN), the Au NPs' formation is negligible.